Rubber composition and tire obtained from the same

ABSTRACT

The present invention provides a rubber composition comprising 100 parts by mass of a rubber and 0.1 to 100 parts by mass of carbon fibers as a filler which are produced by a vapor phase epitaxial growth method and respectively include one or more bottomless cup-shaped carbon network layers, wherein the respective carbon network layer in the form of stacked cups have a large diameter portion whose end face is exposed to a periphery of the carbon fiber. The rubber composition and tire of the present invention have a high thermal conductivity and are excellent in other properties such as mechanical properties.

TECHNICAL FIELD

The present invention relates to a rubber composition and a tire usingthe rubber composition, and more particularly to a rubber compositionhaving an excellent thermal conductivity and a tire using the rubbercomposition.

BACKGROUND ARTS

Various products such as electric and electronic parts, tires and beltshave been produced from a rubber composition containing various naturalrubbers or various synthetic rubbers as a base material thereofaccording to aimed properties thereof. The properties and functions ofsuch products are largely influenced by not only a rubber material asthe base material but also secondary materials such as various fillersblended therein or vulcanization conditions.

For example, as fillers for reinforcing the natural rubbers, there areextensively known carbon black and silica. Further, for the purpose ofenhancing a thermal conductivity of the rubber products, there is knownthe method of blending alumina, boron nitride, etc., therein. Inaddition, for the purpose of imparting good electric properties to therubber products, there is known the method of blending metal powder suchas alumina powder and nickel powder, or electrically conductive carbontherein.

However, in order to obtain high effects by the conventionally knownfillers, it has been inevitably required to increase the amount of thesefillers blended in the rubber products. As a result, the fillers fail tobe uniformly dispersed in rubbers, so that there arise various defectssuch as uneven properties, deteriorated moldability due to increasedviscosity and deteriorated physical properties, as well as unpracticallylow mechanical properties of the resultant rubber products.

Hitherto, there are known rubber compositions containing a rubber andcarbon fibers blended therein (for example, refer to Japanese PatentApplication Laid-open No. Hei 9-157404). However, these conventionalcarbon fibers obtained by a vapor phase epitaxial growth method are inthe form of short fibers that are produced by growing carbon obtained byheat-decomposing hydrocarbons such as benzene and methane at atemperature of about 700 to 1,000° C. over catalyst particles such asultrafine iron or nickel particles as a nucleus. These carbon fibers areusually composed of concentrically grown ordinary carbon network layersor carbon network layers grown in the direction perpendicular to thefiber axis. However, these conventionally known carbon network layersconcentrically grown or grown in the direction perpendicular to thefiber axis tend to be generally deteriorated in adhesion property torubbers.

An object of the present invention is to find out a filler for a rubbercomposition which is capable of imparting a high thermal conductivity torubbers even when used in a relatively small amount, and has no adverseinfluences on other properties such as mechanical properties of rubbers;provide a rubber composition containing such a filler; and provide atire using the rubber composition.

DISCLOSURE OF THE INVENTION

As a result of extensive researches in view of the above object, thepresent inventors have found that the above object can be achieved by:

(1) A rubber composition comprising 100 parts by mass of a rubber and0.1 to 100 parts by mass of carbon fibers as a filler which are producedby a vapor phase epitaxial growth method and respectively include one ormore bottomless cup-shaped carbon network layers, wherein the respectivecarbon network layers in the form of stacked cups have a large diameterportion whose end face is exposed onto a periphery of the carbon fibers.

(2) The rubber composition according to the above aspect (1), whereinsaid filler includes the carbon fibers produced by a vapor phaseepitaxial growth method which respectively include a plurality of thebottomless cup-shaped carbon network layers stacked on each other.

(3) The rubber composition according to the above aspect (2), whereinsaid carbon fibers have a knotless (bridge-free) hollow shape.

(4) The rubber composition according to the above aspect (3), whereinthe hollow carbon fibers include a hollow portion having outside andinside surfaces onto which end faces of the carbon network layer areexposed.

(5) The rubber composition according to the above aspect (4), wherein 2%or more of the end face of the carbon network layer which is located onthe outside surface of the hollow portion of the carbon fiber, isexposed thereto .

(6) The rubber composition according to the above aspect (4) or (5),wherein the exposed end face of the carbon network layer hasirregularities on a scale of atomic level.

(7) The rubber composition according to any of the above aspects (1) to(6), wherein the carbon network layer remains ungraphitized even when itis heat-treated at a high temperature of 2,500° C. or higher.

(8) The rubber composition according to any of the above aspects (1) to(7), wherein the carbon fibers have diameters of 1 to 1,000 nm andlengths of 0.1 to 1,000 μm.

(9) The rubber composition according to any of the above aspects (1) to(7), wherein the carbon fibers have diameters of 5 to 500 nm and lengthsof 0.5 to 750 μm.

(10) The rubber composition according to any of the above aspects (1) to(7), wherein the carbon fibers have diameters of 10 to 250 nm andlengths of 1 to 500 μm.

(11) The rubber composition according to any of the above aspects (1) to(10), further comprising a filler other than the carbon fibers in anamount of 1 to 60 parts by mass.

(12) The rubber composition according to the above aspect (ii), whereinthe filler other than the carbon fibers is carbon black and/or aninorganic filler.

(13) A tire using the rubber composition as claimed in any of the aboveaspects (1) to (12).

The present invention has been accomplished on the basis of thefindings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a carbon fiber according to thepresent invention obtained by a vapor-phase epitaxial growth methodwhich includes a plurality of bottomless cup-shaped carbon networklayers stacked on each other.

FIG. 2 is a transmission electron photomicrograph of a carbon fiberhaving a herring bone structure which was produced by a vapor-phaseepitaxial growth method in Example 1.

FIG. 3 is an enlarged view of the photomicrograph shown in FIG. 2.

FIG. 4 is a schematic view of FIG. 3.

FIG. 5 is a transmission electron photomicrograph of a carbon fiberhaving a herring bone structure which was obtained by heat-treating thecarbon fiber shown in FIG. 2 at a temperature of about 530° C. inatmospheric air for one hour.

FIG. 6 is an enlarged view of the photomicrograph shown in FIG. 5.

FIG. 7 is a further enlarged view of FIG. 6.

FIG. 8 is a schematic view of FIG. 7.

FIG. 9 is a view showing Raman spectra of carbon fibers obtained afterheat-treating a carbon fiber having a herring bone structure (specimenname: Pristine 24PS) at temperatures of 500° C., 520° C., 530° C. and540° C., respectively, in atmospheric air for one hour.

FIG. 10 is a view showing Raman spectra of carbon fibers havingdifferent outer diameters from each other prior to the heat treatment inwhich Pristine 19PS had an average outer diameter of 150 nm, andPristine 24PS had an average outer diameter of 100 nm.

FIG. 11 is a view showing Raman spectra of carbon fibers obtained aftersubjecting Pristine 19PS and Pristine 24PS to ordinary graphitizationtreatment at 3,000° C.

FIG. 12 is a graph showing a distribution of lengths of the carbonfibers at every elapsed time while they were subjected to grinding byball milling.

FIG. 13 is electron photomicrographs of the carbon fiber beforeinitiation of the ball milling.

FIG. 14 is electron photomicrographs of the carbon fiber after theelapse of 2 hr from initiation of the ball milling.

FIG. 15 is electron photomicrographs of the carbon fiber after theelapse of 5 hr from initiation of the ball milling.

FIG. 16 is electron photomicrographs of the carbon fiber after theelapse of 10 hr from initiation of the ball milling.

FIG. 17 is electron photomicrographs of the carbon fiber after theelapse of 24 hr from initiation of the ball milling.

FIG. 18 is a transmission electron photomicrograph showing the state inwhich the cup-shaped carbon network layers began to be liberated fromthe carbon fiber during the ball milling.

FIG. 19 is an enlarged view of FIG. 18.

FIG. 20 is a further enlarged view of FIG. 19.

FIG. 21 is a transmission electron photomicrograph showing the state inwhich the carbon fiber was separated and cut into carbon fibers eachcomposed of several tens of stacked bottomless cup-shaped carbon networklayers.

EXPLANATION OF REFERENCE NUMERALS

1: Bottom portion; 2: End face of a large diameter portion of respectivecup-shaped carbon network layers which are stacked on each other alongan outer periphery of the carbon fiber; 3: Outside surface of a hollowportion of the carbon fiber; 4: Inside surface of a hollow portion ofthe carbon fiber; 10: Carbon network layer; 12: Deposit layer; 14:Central bore; 16: Irregularity on a scale of atomic level.

PREFERRED EMBODIMENT TO CARRY OUT THE INVENTION

The rubber composition of the present invention comprises a rubber as abase material, and carbon fibers added thereto which are produced by avapor phase epitaxial growth method and respectively include one or morebottomless cup-shaped carbon network layers. Preferably the carbonfibers have a plurality of the bottomless cup-shaped carbon networklayers stacked on each other, wherein the respective carbon networklayers in the form of stacked cups have a large diameter portion whoseend face is exposed to an outer periphery of the carbon fibers.

In the followings, the features of the carbon fibers used in the presentinvention are described in detail by referring to FIGS. 1 to 21.

FIG. 1 is a schematic view showing a carbon fiber obtained by avapor-phase epitaxial growth method which includes a plurality ofbottomless cup-shaped carbon network layers stacked on each otheraccording to the present invention. The carbon fiber is composed ofstacked carbon network layers in which a bottom portion represented byreference numeral 1 is lacking or opened. Thus, the carbon fibercomposed of the stacked bottomless carbon network layers has a knotless(bridge-free) hollow shape. A portion represented by reference numeral 2is an end face of a large diameter portion of the respective cup-shapedcarbon network layers stacked on each other along an outer periphery ofthe carbon fibers. In the present invention, the end face is in anexposed state as described below. Further, portions represented byreference numerals 3 and 4 are respectively an outside surface and aninside surface of a hollow portion of the carbon fiber onto which theend face of the respective carbon network layers is preferably exposed.

Next, FIG. 2 is a transmission electron photomicrograph of a carbonfiber having a herring bone structure which was produced by avapor-phase epitaxial growth method; FIG. 3 is an enlarged view of thephotomicrograph shown in FIG. 2; and FIG. 4 is a schematic view of thecarbon fiber having a herring bone structure. Here, reference numerals10 and 14 represent the inclined carbon network layers and a centralbore of the carbon fiber, respectively. A deposit layer 12 made ofexcessive amorphous carbon is formed on the carbon fiber so as to coverthe carbon network layers 10.

Upon production of carbon fibers by the above vapor-phase epitaxialgrowth method, by controlling the vapor-phase epitaxial growthconditions such as catalysts, temperature range and flow rate, it ispossible to produce carbon fibers having a herring bone structure inwhich stacked carbon network layers are inclined at a constant anglerelative to an axis of the carbon fiber. The carbon fibers of thepresent invention have such a herring bone structure.

On the surfaces of the carbon fibers produced by the vapor-phaseepitaxial growth method, there is usually formed a thin deposit layermade of excessive amorphous carbon which has failed to be sufficientlycrystallized. It is considered that the deposit layer has a low activityand, therefore is deteriorated in adhesion to rubbers.

The carbon fibers of the present invention are characterized in that thedeposit layer 12 covering the carbon network layers 10 is partiallyremoved so as to expose at least a part of the end face (end of6-membered ring) of the respective carbon network layers. The thusexposed end face of the carbon network layer 10 tends to be bonded toother atoms and exhibit an extremely high activity. In the carbon fibersof the present invention, at least 2% and preferably 7% or higher of theend face (end of 6-membered ring) of the respective carbon networklayers is exposed. As a result, the carbon fibers of the presentinvention can exhibit an improved adhesion to rubbers, and can provide arubber composition having an excellent thermal conductivity. Therefore,from the above viewpoints, the degree of exposure of the end face of therespective carbon network layers is preferably as large as possible.

Also, in the carbon fibers of the present invention, when the depositlayer is positively removed therefrom, the exposed portion of the endface of the carbon network layers can be further increased, therebyenabling production of a rubber composition having an extremelyexcellent thermal conductivity. The reason therefor is considered asfollows. That is, the deposit layer 12 is removed by the below-mentionedheat treatment in atmospheric air, etc., and at the same time, thenumber of oxygen-containing functional groups such as phenolic hydroxylgroups, carboxyl groups, quinone-type carbonyl groups and lactone groupswhich are present on the exposed end face of the carbon network layersis increased, so that the carbon fibers can be enhanced inhydrophilicity and affinity to various substances by the increase inthese oxygen-containing functional groups.

Various methods may be used to remove the deposit layer 12 and exposethe end face of the carbon network layers 10. One of the methods is sucha method in which the carbon fibers are heated at a temperature of 400°C. or higher, preferably 500° C. or higher and more preferably 520 to530° C. in atmospheric air for one to several hours to oxidize andthermally decompose the deposit layer 12. Alternatively, the depositlayer 12 may be removed to expose the end face of the carbon networklayers by the method of washing the carbon fibers with supercriticalwater, the method of heating the carbon fibers immersed in hydrochloricacid or sulfuric acid to a temperature of about 80° C. while stirringwith a stirrer.

FIG. 5 is a transmission electron photomicrograph of a carbon fiberobtained by heat-treating the carbon fiber having a herring bonestructure shown in FIG. 1 at a temperature of about 530° C. for one hourin atmospheric air; FIG. 6 is an enlarged view of the photomicrographshown in FIG. 5; FIG. 7 is a further enlarged view of FIG. 6; and FIG. 8is a schematic view of FIG. 7. It is recognized that when subjecting thecarbon fiber to the above heat treatment, a part of the deposit layer 12is removed, so that the end face (end of carbon 6-membered ring) of therespective carbon network layers 10 is exposed. Meanwhile, it isconsidered that most of the residual deposit layer 12 is decomposed andsimply attached onto the carbon fibers. Therefore, when the carbonfibers are heat-treated for several hours, and further washed withsupercritical water, 100% of the deposit layer 12 can be completelyremoved therefrom.

FIG. 9 shows Raman spectrum of the carbon fiber (specimen name: Pristine24PS; average outer diameter: 100 nm) which was allowed to stand inatmospheric air for one hour, as well as those obtained afterheat-treating the carbon fibers at temperatures of 500° C., 520° C.,530° C. and 540° C., respectively, in atmospheric air for one hour. Asis apparent from FIG. 9, since the peaks of the spectra were observed at1,360 cm⁻¹ and 1,580 cm⁻¹, it was confirmed that these fibers werecarbon fibers having no graphitization structure.

As shown in FIGS. 5 to 8, the respective carbon fibers of the presentinvention are composed of one or more bottomless cup-shaped carbonnetwork layers, and usually several to several hundred thousand carbonnetwork layers are stacked on each other. The individual carbon fibersare in the form of fine particles and, therefore, extremely excellent indispersibility in rubbers or resins. In particular, when the carbonfibers are used in rubbers to form a composite material therewith, theobtained rubber composition can exhibit not only a good flexibility anda high strength, but also a high adhesion to the rubbers and anexcellent thermal conductivity.

In addition, the respective carbon fibers of the present invention arepreferably in the form of a knotless (bridge-free) hollow shape, andmore preferably have a hollow portion extending at least in the range offrom several ten nm to several ten μm as shown in FIGS. 5 to 8. Further,in the hollow carbon fibers, the end face of the respective carbonnetwork layers is preferably exposed onto outside and inside surfaces ofthe hollow portion of the carbon fibers. The degree of exposure of theend face onto the outside and inside surfaces of the hollow portion ismore preferably as large as possible. Among them, the percentage ofexposure of the end face onto the outside surface of the hollow portionof the carbon fiber is preferably 2% or higher and more preferably 7% orhigher.

Meanwhile, the carbon network layers are inclined at an angle of about20 to 35° relative to the center line of the respective carbon fibers.

Further, in the carbon fibers of the present invention, the exposed endface of the carbon network layers preferably have irregularities on ascale of atomic level. The irregularities formed on the end face have ananchoring effect and allows the carbon fibers to exhibit a moreexcellent adhesion to the rubbers, so that the resultant rubbercomposition can show extremely excellent thermal properties.

It is considered that the irregularities on a scale of atomic level asformed on the end face are attributed to a turbostratic structure causedby slippage (grind) of carbon network surfaces. In the carbon fibershaving the turbostratic structure, the respective carbon hexagonalnetwork surfaces constitute a parallel stacked structure. In the stackedstructure, the respective carbon hexagonal net surfaces are slipped orrotated in a plane direction thereof, so that the structure shows nocrystallographic regularity.

In addition, the carbon fibers of the present invention are notgraphitized even when being heat-treated at a high temperature of 2,500°C. or higher, while ordinary carbon fibers are graphitized when beingheat-treated at such a high temperature. The reason why the carbonfibers of the present invention are not graphitized even when beingsubjected to such a graphitization treatment is considered as follows.That is, the deposit layer 12 which is susceptible to graphitization isremoved from the carbon fibers, and the residual portion of the carbonfibers with the herring bone structure which is obtained after removingthe deposit layer 12 therefrom is inherently unsusceptible tographitization. This indicates that the carbon fibers of the presentinvention is thermally stable.

Meanwhile, FIG. 10 shows Raman spectra of carbon fibers with a herringbone structure having different outer diameters from each other prior tothe heat treatment in which Pristine 19PS had an average outer diameterof 150 nm, and Pristine 24PS had an average outer diameter of 100 nm. Onthe other hand, FIG. 11 shows Raman spectra of carbon fibers obtainedafter subjecting the above specimens to ordinary graphitizationtreatment at 3,000° C. From the comparison between both the spectra,since no significant difference was observed therebetween and the peaksthereof were present at 1,360 cm⁻¹ and 1,580 cm⁻¹, it is confirmed thatthe carbon fibers of the present invention are not graphitized even whenbeing subjected to ordinary graphitization treatment.

The carbon fibers of the present invention preferably have diameters of1 to 1,000 nm, more preferably 5 to 500 nm and most preferably 10 to 250nm, and further preferably have lengths of 0.1 to 1,000 μm, morepreferably 0.5 to 750 μm and most preferably 1 to 500 μm. When thediameters and lengths of the carbon fibers are controlled within theabove-specified ranges, the carbon fibers can be enhanced in affinity torubbers which is useful to obtain a rubber composition having animproved thermal conductivity.

The carbon fibers of the present invention may be produced, for example,by the following method.

Hydrocarbons such as benzene are charged under a predetermined partialpressure into an ordinary reaction vessel, and reacted therein at atemperature of about 1,100° C. for about 20 min in the presence of atransition metal complex such as ferrocene as a catalyst to obtaincarbon fibers with a herring bone structure which have a diameter ofabout 100 nm. In this case, when appropriately controlling a flow rateof the raw material and the reaction temperature, it is possible toproduce carbon fibers which are respectively composed of bottomlesscup-shaped carbon network layers stacked on each other, and have aknotless (bridge-free) hollow structure extending over the range of fromseveral ten nm to several ten μm. As described above, since the thusobtained carbon fibers have a deposit layer thereon, at least a part ofthe deposit layer is removed by the above method. The thus producedcarbon fibers are short fibers (having a length of several ten μm) whichare composed of several ten thousand to several hundred thousand unitcarbon network layers each having a bottomless cup shape, i.e., abottom-opened reverse V-shape in cross-section. The short fibers have alarge molecular weight (length) and are insoluble in water, organicsolvents, etc. The carbon fibers of the present invention are obtainedby cutting these short fibers into individual carbon fibers composed ofone or more unit carbon network layers and preferably several to severalten thousand unit carbon network layers.

As the method of obtaining the carbon fibers of the present invention bycutting the above short fibers into individual carbon fibers, there maybe used various methods, for example, there may be suitably used themethod of adding an appropriate amount of water or a solvent to theshort fibers and then moderately grinding the short fibers in a mortarwith a pestle for an appropriate time.

With the above procedure, since the cyclic carbon network layers have arelatively high strength and are bonded to each other by a weak van derWaals force, the short fibers are cut and separated into individualcarbon fibers especially at a weakly bonding portion between the carbonnetwork layers without breakage of the respective unit carbon networklayers.

Meanwhile, the short fibers may be effectively cut into individualcarbon fibers by grinding the fibers using a mortar in a liquidnitrogen. When the liquid nitrogen is evaporated, water present inambient air is absorbed therein and formed into ice. Therefore, theshort fibers are ground by the pestle together with the ice, so thatmechanical stress applied thereto is reduced, resulting in facilitatedseparation between the unit carbon network layers.

The above cutting step may be performed prior to removal of the depositlayer. In reverse, after completion of the cutting step, the depositlayer may be removed from the obtained carbon fibers.

From the industrial viewpoints, the above cutting step may also besuitably performed by subjecting the short fibers to grinding treatmentby ball milling. In the followings, the method of controlling a lengthof the carbon fibers by ball milling is explained in detail. In thismethod, there may be used a ball mill, for example, available from AsahiRika Seisakusho Co., Ltd., and alumina balls each having a diameter of 5mm. More specifically, for example, 1 g of the above carbon fibers, 20 gof alumina balls and 50 cc of distilled water may be charged into acell, and milled therein at a rotating speed of 350 rpm. The carbonfibers subjected to the cutting treatment by the above method weresampled at every elapsed time of 1, 3, 5, 10 and 24 hr, and analyzedusing a laser particle size distribution meter. The results are shown inFIG. 12 as a distribution of lengths of the carbon fibers sampled atevery elapsed time. As a result, it is confirmed that as the elapsedmilling time was increased, the fiber length was shortened. Inparticular, after the elapsed time of 10 hr, the fiber length wasabruptly reduced to 10 μm or lower. Also, after the elapsed time of 24hr, another peak was observed at about 1 μm, and the fiber length wasstill finer. Meanwhile, at the peak observed at about 1 μm, the lengthand diameter of the carbon fibers are almost identical to each other.Therefore, it is considered that the length and diameter are counteddouble.

Also, FIG. 13 is a transmission electron photomicrograph of the carbonfiber before initiation of the ball milling, and FIGS. 14, 15, 16 and 17are transmission electron photomicrographs of the carbon fibers afterthe elapse of 2 hr, 5 hr, 10 hr and 24 hr, respectively, from initiationof the ball milling. The carbon fibers before the ball milling arecomposed of those fibers having a length of several ten μm which areentangled with each other, and, therefore, have an extremely low bulkdensity. Whereas, as the elapsed milling time is increased, the fiberlength is reduced. As a result, it is recognized that after the elapsedtime of 24 hr, the carbon fibers are substantially in the form ofparticles. Also, after the elapsed time of 24 hr, the carbon fibers aresubstantially free from entanglement and, therefore, have a high bulkdensity.

In addition, FIG. 18 is a transmission electron photomicrograph showingthe condition in which the carbon fibers are cut into individual carbonfibers during the ball milling. FIGS. 19 and 20 are enlarged views ofthe photomicrograph shown in FIG. 18. As is apparent from these views,it is recognized that the carbon fibers are cut into individual carbonfibers by not breakage of the fibers but release of the bottomlesscup-shaped carbon network layers from the carbon fibers.

Next, FIG. 21 is a transmission electron photomicrograph showing thecarbon fiber whose length is controlled to the above-described conditionin which several tens of the bottomless cup-shaped carbon network layersare stacked on each other. The carbon fibers are about 60 nm in bothlength and diameter thereof, and have a thin-wall tubular shape with alarge cavity, i.e., a knotless (bridge-free) hollow shape, and the endface of the respective carbon network layers is exposed onto the outsideand inside surfaces of the hollow portion of the respective carbonfibers. Thus, it is recognized that the bottomless cup-shaped carbonnetwork layers are released and separated from the carbon fibers, andthe respective carbon network layers are free from breakage of the shapethereof. Meanwhile, the length of the carbon fibers may be optionallycontrolled by varying the milling conditions. Thus, the millingconditions can be suitably controlled so as to adjust the diameter andlength of the obtained carbon fibers to the above preferred ranges ofthe present invention.

Meanwhile, when ordinary concentric carbon nanotubes are subjected togrinding treatment, there occur problems such as breakage of the tubes,formation of cracks extending in an axial direction on an outer surfacethereof, formation of burrs as well as so-called coreless condition,resulting in difficulty in controlling the length of the carbon fibers.

Examples of the rubbers used in the present invention include naturalrubbers; general synthetic rubbers; diene-based special rubbers such asemulsion-polymerized styrene-butadiene rubbers, solution-polymerizedstyrene-butadiene rubbers, high-cis-1,4-polybutadiene rubbers,low-cis-1,4-polybutadiene rubbers and high-cis-1,4-polyisoprene rubbersfor example; olefin-based special rubbers such as nitrile rubbers,hydrogenated nitrile rubbers and chloroprene rubbers for example; otherspecial rubbers such as ethylene-propylene rubbers, butyl rubbers,halogenated butyl rubbers, acrylic rubbers and chlorosulfonatedpolyethylene for example; hydrin rubbers; fluorine rubbers; polysulfiderubbers and urethane rubbers for example. Among these rubbers, in viewof good balance between costs and performance, preferred are naturalrubbers and general synthetic rubbers.

The rubber composition of the present invention is preferably vulcanizedupon use by the method of adding sulfur, peroxides, metal oxides, etc.,to the composition and heating the resultant mixture to crosslinking thecomposition, the method of irradiating light to the composition to whicha photopolymerization initiator is added for crosslinking thecomposition, and the method of irradiating an electron beam or aradiation to the composition for crosslinking the composition.

The rubber composition of the present invention contains 100 parts bymass of a rubber and 0.1 to 100 parts by mass of the above carbon fibersblended in the rubber. When the content of the carbon fibers lies withinthe above range, the resultant rubber composition shows a sufficientthermal conductivity as well as a good workability upon mixing ormolding. Further, from the same view points, the content of the carbonfibers in the rubber composition is more preferably in the rang e of 0.5to 50 parts by mass.

The rubber composition of the present invention may suitably contains,in addition to the above carbon fibers, carbon black and/or an inorganicfiller and various other fillers in an amount of 1 to 60 parts by massand preferably 1 to 40 parts by mass. The rubber composition containingan appropriate amount of these fillers can exhibit a higher reinforcingeffect as compared to the rubber composition to which the carbon fibersonly are added.

The carbon black blended in the rubber composition is not particularlylimited, and may be optionally selected from those generally used asreinforcing fillers for conventional rubber compositions. Specificexamples of the carbon black include FEF, SRF, HAF, ISAF and SAF. Thecarbon black preferably has an iodine absorption (IA) of 60 mg/g orhigher and a dibutyl phthalate oil absorption (DBP) of 80 mL/100 g orhigher. Of these carbon blacks, preferred are HAF, ISAF and SAF havingan excellent abrasion resistance.

As the inorganic filler, there may be used those inorganic fillersconventionally used in rubber industries without any particularlimitation. Examples of the inorganic filler include alumina (Al₂O₃)such as γ-alumina and α-alumina, alumina monohydrate (Al₂O₃.H₂O) such asboehmite and diaspore, aluminum hydroxide [Al(OH)₃] such as gibbsite andbayerite, aluminum carbonate [Al₂(CO₃)₃], magnesium hydroxide [Mg(OH)₂],magnesium oxide (MgO), magnesium carbonate (MgCO₃), talc(3MgO.4SiO₂.H₂O), attapulgite (5MgO.8SiO₂.9H₂O), titanium white (TiO₂),titanium black (TiO_(2n-1)), calcium oxide (CaO), calcium hydroxide[Ca(OH)₂], magnesium aluminum oxide (MgO.Al₂O₃), clay (Al₂O.₃.2SiO₂),kaolin (Al₂O₃.2SiO₂.2H₂O), pyrophyllite (Al₂O₃.4SiO₂.H₂O), bentonite(Al₂O₃.4SiO₂.2H₂O), aluminum silicate (Al₂SiO₅, Al₄.3SiO₄.5H₂O, etc.),magnesium silicate (Mg₂SiO₄, MgSiO₃, etc.), calcium silicate (Ca₂.SiO₄,etc.), aluminum calcium silicate (Al₂O₃.CaO.2SiO₂, etc.), magnesiumcalcium silicate (CaMgSiO₄), calcium carbonate (CaCO₃), zirconium oxide(ZrO₂), zirconium hydroxide [ZrO(OH)₂.nH₂O], zirconium carbonate(ZrCO₃), and crystalline aluminosilicates containing hydrogen, alkalimetal or alkali earth metal acting for correcting electric charges, suchas various zeolites. Of these inorganic fillers, preferred are silicaand aluminum hydroxide having a nitrogen adsorption specific surfacearea (N₂SA) of 1 to 20 m²/g, and more preferred is silica.

The silica usable in the present invention may be optionally selectedfrom those generally used as reinforcing materials for conventionalrubbers without any particular limitation. Specific examples of thesilica include wet silica (hydrous silica), dry silica (anhydroussilica), calcium silicate and aluminum silicate. Of these materials,preferred is synthetic silica obtained by a precipitation method.

Meanwhile, the rubber composition may be mixed and molded by knownmethods ordinarily used for mixing and molding rubbers, without anyparticular limitation.

The rubber composition of the present invention which contains a smallamount of the above carbon fibers can be considerably enhanced inthermal conductivity without any significant change in other physicalproperties and deterioration in moldability. Therefore, the rubbercomposition of the present invention can be extensively used in variousapplications such as electric and electronic parts, tires, belts andvarious other products. Meanwhile, the rubber composition of the presentinvention may appropriately contain various additives generally used inrubber industrial fields such as, for example, vulcanizationaccelerators, reinforcing materials, anti- aging agents, softeningagents and ordinary additives for rubbers.

EXAMPLES

The present invention will be described in more detail below withreference to the following examples. However, these examples are onlyillustrative and not intended to limit the invention thereto.

[Method for Evaluation of Physical Properties]

The thermal conductivity of each of the rubber sheets obtained inExamples 1 to 6 and Comparative Example 1 was evaluated. Morespecifically, the thermal conductivity was evaluated by the valuemeasured by a rapid thermal conductivity meter “QTM-500” available fromKyoto Denshi Co., Ltd.

Example 1

Benzene as a raw material was charged into a chamber of a reactor undera partial pressure corresponding to a vapor pressure thereof at 20° C.at a flow rate of 0.3 L/h in a hydrogen gas flow. Ferrocene as acatalyst was vaporized at 185° C., and charged into the chamber at aconcentration of 3×10⁻⁷ mol/s. The reaction was conducted at 1,100° C.for 20 min to obtain cup stack-type carbon fibers having a diameter ofabout 100 nm which were composed of stacked bottomless cup-shaped carbonnetwork layers having a herring bone structure (hereinafter referred toas “carbon fibers A”).

The carbon fibers A and various additives were blended in natural rubber(NR) at ratios shown in Table 1, and the resultant mixture was kneadedunder the following kneading conditions, and then formed into a sheetunder the following sheet-forming conditions, thereby obtaining a sheetmade of the vulcanized rubber composition. Meanwhile, all of the amountsof the materials blended as shown in Table 1 represent “part(s) bymass”. The measured thermal conductivity values are shown in Table 1.

(1) Kneading Conditions

The natural rubber (NR) was simply kneaded at a temperature of 70° C.and a rotating speed of 50 rpm for 3 min using a Laboplastomillavailable from Toyo Seiki Co., Ltd. Then, the respective additives asshown in Table 1 except for the vulcanization accelerator and sulfurwere charged into the mill, and the contents in the mill were furthermixed together at a temperature of 70° C. and a rotating speed of 50 rpm(nonproductive mixing). The resultant mixture was taken out of the mill,cooled and weighed. Thereafter, the remaining vulcanization acceleratorand sulfur were added to the mixture, and the obtained mixture was mixedtogether again at a temperature of 50° C. and a rotating speed of 50 rpmusing a Brabender (productive mixing).

(2) Sheet-Forming Conditions

The thus kneaded mixture was vulcanized at 150° C. for 15 min using ahigh-temperature press to prepare a 1 mm-thick vulcanized rubber sheet.

Example 2

The same procedure as in EXAMPLE 1 was repeated except that the carbonfibers A and various additives were blended at ratios shown in Table 1,thereby obtaining a sheet made of the vulcanized rubber composition. Theevaluation results are shown in Table 1.

Example 3

The same procedure as in EXAMPLE 1 was repeated except that the cupstack-type carbon fibers composed of stacked bottomless cup-shapedcarbon network layers which were previously subjected to disentanglementtreatment (hereinafter referred to as “carbon fibers B”) were used,thereby obtaining a sheet made of the vulcanized rubber composition. Theevaluation results are shown in Table 1.

Example 4

The same procedure as in EXAMPLE 1 was repeated except that the carbonfibers B and various additives were blended at ratios shown in Table 1,thereby obtaining a sheet made of the vulcanized rubber composition. Theevaluation results are shown in Table 1.

Examples 5 and 6

The carbon fibers A were cut into individual carbon fibers by the abovecutting step, thereby obtaining cup-stack-type carbon fibers having adiameter of 50 to 200 nm and a length of 0.05 to 10 μm which werecomposed of stacked bottomless cup-shaped carbon network layers(hereinafter referred to as “carbon fibers C”). The same procedure as inEXAMPLE 1 was repeated except that the carbon fibers C and variousadditives were blended at ratios shown in Table 1, thereby obtaining asheet made of the vulcanized rubber composition. The evaluation resultsare shown in Table 1.

Comparative Example 1

The same procedure as in EXAMPLE 1 was repeated except that no carbonfibers were added, and various additives were blended at ratios shown inTable 1, thereby obtaining a sheet made of the vulcanized rubbercomposition. The evaluation results are shown in Table 1. TABLE 1 Com.Examples Ex. 1 2 3 4 5 6 1 NR 100 100 100 100 100 100 100 HAF-gradecarbon 25 — 25 — 25 — 50 black Cup stack-type carbon 25 50 — — — — —nanofiber A*¹ Cup stack-type carbon — — 25 50 — — — nanofiber A*² Cupstack-type carbon — — — — 25 50 — nanofiber A*³ Anti-aging agent 2 2 2 22 2 2 Stearic acid 2 2 2 2 2 2 2 Zinc oxide 3 3 3 3 3 3 3 Vulcanization1 1 1 1 1 1 1 accelerator Sulfur 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Thermalconductivity 0.38 0.46 0.39 0.49 0.38 0.46 0.33 (W/mK)Note:^(1*)Carbon fiber A; Diameter: 50 to 200 nm; Length 5 to 200 μm^(2*)Carbon fiber B; Diameter: 50 to 200 nm; Length 5 to 200 μm;Previously subjected to disentanglement^(3*)Carbon fiber C; Diameter: 50 to 200 nm; Length 0.05 to 10 μm

In the carbon fibers produced by a vapor-phase epitaxial growth methodaccording to the present invention, the end face of a large diameterportion of the stacked cup-shaped carbon network layers is exposed ontoan outer periphery of the carbon fibers. For this reason, since theexposed end face has a high activity, the carbon fibers are excellent inadhesion to a rubber material in a rubber composition and, therefore,usable as a suitable raw material for production of the rubbercomposition. Further, since the end faces of the respective inclinedbottomless cup-shaped carbon network layers having a herring bonestructure are exposed in a layered form, the activity of the exposed endfaces (ends of 6-membered ring) of the carbon network layers can beextremely enhanced. Therefore, the carbon fibers are excellent inadhesion to rubbers, so that it is possible to provide a material forthe rubber composition having an excellent thermal conductivity. Inaddition, the end faces of the respective carbon network layers whichare exposed on the surface of the carbon fibers in a layered form byremoving the deposit layer therefrom, are not aligned with each otherand, therefore, have irregularities on a scale of atomic level. As aresult, the carbon fibers can exhibit an anchoring effect to rubbers orrubber products and, therefore, are more excellent in adhesion torubbers, thereby providing a rubber composition having an extremelyexcellent thermal conductivity.

According to the rubber composition of the present invention, even whenthe carbon fibers are added in a small amount, the resultant rubbercomposition can be considerably improved in thermal conductivity withoutany significant change in various other physical properties as well asdeterioration in moldability. Accordingly, the vulcanized rubbercomposition of the present invention can be extensively applied toelectric and electronic parts, tires, belts and various other products.In particular, when the rubber composition is applied to tires, it ispossible to prevent heat generation therefrom owing to its good heatdissipation effect.

1. A rubber composition comprising 100 parts by mass of a rubber and 0.1to 100 parts by mass of carbon fibers as a filler which are produced bya vapor phase epitaxial growth method and respectively include one ormore bottomless cup-shaped carbon network layers, wherein the respectivecarbon network layer in the form of stacked cups have a large diameterportion whose end face is exposed onto a periphery of the carbon fiber.2. The rubber composition according to claim 1, wherein said fillerincludes the carbon fibers produced by a vapor phase epitaxial growthmethod which respectively include a plurality of the bottomlesscup-shaped carbon network layers stacked on each other.
 3. The rubbercomposition according to claim 2, wherein said carbon fiber has aknotless (bridge-free) hollow shape.
 4. The rubber composition accordingto claim 3, wherein the hollow carbon fibers include a hollow portionhaving outside and inside surfaces onto which end faces of the carbonnetwork layer are exposed.
 5. The rubber composition according to claim4, wherein 2% or more of the end face of the carbon network layer whichis located on the outside surface of the hollow portion of the carbonfiber, is exposed thereto.
 6. The rubber composition according to claim4, wherein the exposed end face of the carbon network layer hasirregularities on a scale of atomic level.
 7. The rubber compositionaccording to claim 1, wherein the carbon network layer remainsungraphitized even when it is heat-treated at a high temperature of2,500° C. or higher.
 8. The rubber composition according to claim 1,wherein the carbon fibers have diameters of 1 to 1,000 nm and lengths of0.1 to 1,000 μm.
 9. The rubber composition according claim 1, whereinthe carbon fibers have diameters of 5 to 500 nm and lengths of 0.5 to750 μm.
 10. The rubber composition according claim 1, wherein the carbonfibers have diameters of 10 to 250 nm and lengths of 1 to 500 μm. 11.The rubber composition according claim 1, further comprising a fillerother than the carbon fibers in an amount of 1 to 60 parts by mass. 12.The rubber composition according to claim 11, wherein the filler otherthan the carbon fibers is carbon black and/or an inorganic filler.
 13. Atire using the rubber composition as claimed in claim
 1. 14. The rubbercomposition according to claim 5, wherein the exposed end face of thecarbon network layer has irregularities on a scale of atomic level.